Inswinger and outswinger controversy
= Introduction = One of the big controversies surrounding the study of late Roman (usually metal-framed ballistas) is how their arms operate. There have traditionally been two camps: those who claim these metal-framed ballistas were outswingers and those who claim they were inswingers. In an outswinger the arms of the ballista point away from the case and slider in ~80 degree angle before pulling back the bowstring. When the bowstring is pulled back, the tips of the arm rotate inwards, towards the side of the case and slider. In an inswinger the arms initially point towards the target and are pulled back through the field-frame, tips of the arms almost touching each other in the process. As mentioned here, most of existing research leans heavily on interpretation of ambiguous ancient texts and diagrams. The inswinger theories, on the other hand, lean heavily on archaeological finds. = Operating characteristics of an inswinger = If the arms of a ballista are inswinging, they can travel a much wider arc (~115 degrees) than in an outswinging configuration (~65 degrees). This means that the torsion bundles in an inswinger can store nearly twice the amount of energy, if the bundle material can take it. Also, the bowstring moves quite differently in an inswinger. Let's take an inswinger CAD model built according to P.H.'s dimensions as an example: * When arm rotates from 0 to 25 degrees, the string moves 4,7cm forward * When arm rotates from 25 to 50 degrees, the string moves 5,3cm forward * When arm rotates from 50 to 75 degrees, the string moves 2,0cm forward * When arm rotates from 75 to 100 degrees, the string moves 15,0cm forward As the arms themselves have accelerated for the whole duration of the shot, the last fourth (75->100 degrees) takes a lot less time than the first fourth (0->25 degrees). This means that the bowstring's acceleration at the end is ridicuously high. Apparently (=testing needed) this final phase of arm movement is critical for the velocity of the bolt. However, this rapid acceleration of the string (and bolt) comes at the cost of reducing the leverage of the arms. This means the bolts have to be light to benefit. Wilkins has criticized the inswinging theory on several points (2000: 100). First, he claims that the arms (as described by P.H.) have to be shortened or they will clash. This is not true, unless one lengthens the metal hooks of the arms far beyond the wooden arms. P.H. states that the "cone-shaped parts" have to be 11 dactyls long (e.g. Marsden 1971: 217; Wilkins 1995: 32). The little ladder "boards" to which the field-frames are attached are 26 dactyls and 24 dactyls long, respectively. In addition, 2 dactyl long tenons are attached to their ends (Marsden 1971: 215; Wilkins 1995: 28). The tenons were probably made similarly to the flattened and pierced sections of the Orsova kamarion (Baatz 1978: 11) and were attached to the loops in field-frames. In any case the outer edges of the cord bundles are roughly 28 dactyls away from each other. As the arms need to have heels to stop them, they need to be pushed through the cord bundle and beyond. This means that when facing each other (perpendicular to the case and slider) their hooked tips (if 1,5 dactyls long) are ~5 dactyls away from each other. This is more than enough clearance. Wilkins probably had issues because he had lengthened his arm hooks excessively as can be seen in all reconstruction photographs and diagrams (Wilkins 1995 & 2003). As the length of the metal hooks is not defined anywhere, Wilkins' modification was highly arbitrary. From engineering point of view lengthening the metal hooks was also very unwise: it placed too much stress on the end of the metal hook, which would have otherwise been supported by the very rigid wooden arm. Lengthening the hook requires it to be made much thicker and thus heavier. As the arm is only a method by which energy is transmitted to the bowstring and to the bolt, it should be as light as possible. The heavier the ballista arms are, the more energy they waste, especially if their forward momentum is stopped by the heels of the arms and not the bowstring. A taut bowstring would at least transfer some of the wasted energy into the bolt. Wilkins (2000: 100) also claims that in an inswinger the bolt leaves the slider before the arms have moved through their whole arc. His argument is that when the arms are moving outwards (away from the slider) during last half of their travel (in degrees), the forward movement of the string is slowed down and bolt leaves the slider. Wilkins seems to have confused the movement of the arm with the bowstring. It is true that the arms are moving outwards as he says. However, he ignores three important facts: * The arms accelerate until the heels (or bowstring) stops them * The bowstring's (and thus bolt's) acceleration at any given point depends on ** arm acceleration (at that point) ** the amount of leverage given by arm/bowstring geometry (at that point) * Bowstring never decelerates as it retains it's forward momentum - minus some lost to friction - regardless of what the arms are doing At worst, bowstring's acceleration decreases (or stops?) momentarily but the last phase of arm movement (see above) more than makes up for this. = Operating characteristics of an outswinger = = Case for inswingers = Ancient texts TODO: Describe Iriarte's (2003) inswinger theories here. The Hatra ballista The Hatra ballista is probably the most conclusive piece of evidence for inswinger proponents. The thorough description of the find along with a reconstruction was published by Baatz (1978). The Hatra ballista contained 8 close ended bronze corner fittings. Half of these were "mirrored" but otherwise the same as the other half. The fact that these corner fittings were close-ended means that they could only have been attached to the end of a beam (e.g. a side stanchion). The drawings in Baatz's article clearly show that each corner of the Hatra ballista frame had one of these fittings. Furthermore, Wilkins (2003: 68) provides a additional few excavation/conservation photos which too make this clear. In a nutshell, there were two vertical side stanchions at each end of the Hatra ballista frame and the bronze corner fittings were attached to their ends. It is very difficult to explain the corner fittings in any other way. This is what effectively forces the Hatra ballista to be an inswinger rather than outswinger. To circumvent this problem (if he was indeed aware of it) Wilkins (2003: 68) moved the inner side stanchion towards the middle of the frame. This allowed him to force the Hatra ballista to fit his outswinger theory at the cost of ignoring the inner corner fittings which had been attached to vertical beams. And if the vertical beams didn't go from bottom to the top, they serve absolutely no purpose. Other archaeological finds Since Marsden (1971) several field-frames, kamarions and other parts have been found from Gornea, Lyon, Sala and Orsova. Iriarte (2000: 63, 66) tested how these existing kambestria (field-frames) finds would have worked in the outswinger configuration. The results were mixed, with maximum arm travel between 38 and 68 degrees. In an inswinger configuration with same kambestria the arm travel was between 66 and 125 degrees - roughly double that in the inswinger configuration. Although Iriarte (2000) initially reconstructed his cheiroballistra as an outswinger, but later (2003) "changed camps" in his "The Inswinger Theory" article. TODO: Discuss previous cheiroballistra outswinger reconstructions and point out issues that arise when ignoring archaeological finds. = Case for outswingers = = References = References can be found from the bibliography page. Category:backup